A typical lithium-sulphur cell comprises an anode (negative electrode) formed from lithium metal or a lithium metal alloy, and a cathode (positive electrode) formed from elemental sulphur or other electroactive sulphur material. The sulphur or other electroactive sulphur-containing material may be mixed with an electrically conductive material, such as carbon, to improve its electrical conductivity. Typically, the carbon and sulphur are ground and then mixed with a solvent and a binder to form a slurry. The slurry is applied to a current collector and then dried to remove solvent. The resulting structure is calendared to form a composite structure, which is cut into the desired shape to form a cathode. A separator is placed on the cathode and a lithium anode placed on the separator. Electrolyte is then introduced into the assembled cell to wet the cathode and separator.
Lithium-sulphur cells are secondary cells. When a lithium-sulphur cell is discharged, the sulphur in the cathode is reduced in two-stages. In the first stage, the sulphur (e.g. elemental sulphur) is reduced to polysulphide species, Sn2− (n≥2). These species are generally soluble in the electrolyte. In the second stage of discharge, the polysulphide species are reduced to lithium sulphide, Li2S, which, typically, deposits on the surface of the anode.
When the cell is charged, the two-stage mechanism occurs in reverse, with the lithium sulphide being oxidised to lithium polysulphide and thereafter to lithium and sulphur. This two-stage mechanism can be seen in both the discharging and charging profiles of a lithium-sulphur cell. Accordingly, when a lithium-sulphur cell is charged, its voltage typically passes through an inflexion point as the cell transitions between the first and second stage of charge.
Lithium-sulphur cells may be (re)charged by applying an external current to the cell. Typically, the cell is charged to a fixed cut-off voltage of, for example, 2.45 V. However, with repeated cycling over an extended period, the capacity of the cell may fade. Indeed, after a certain number of cycles, it may no longer be possible to charge the cell to the fixed cut-off voltage because of the increasing internal resistance of the cell. By repeatedly charging the cell to the selected cut-off voltage, the cell may eventually be repeatedly over-charged. This can have a detrimental effect on the longevity of the cell, as undesirable chemical reactions may lead to damage to, for example, the cell's electrodes and/or electrolytes.
In view of the foregoing, it is desirable to avoid over-charging the lithium-sulphur cell. WO 2007/111988 describes a process for determining the point at which a lithium sulphur cell becomes fully charged. Specifically, this reference describes adding an N—O additive, such as lithium nitrate, to the electrolyte of the cell. According to the passage at page 16, lines 29 to 31, of this reference, the additive is effective in providing a charge profile with a sharp increase in voltage at the point of full charge. Accordingly, charging can be terminated once this rapid increase in voltage is observed. This sharp increase in voltage occurs once the second stage of charge is complete, at voltages beyond the inflexion point between the two stages of charge described above.
The method of WO 2007/111988 relies on the voltage of the cell increasing very sharply as the cell reaches full capacity. Not all lithium-sulphur cells, however, exhibit such a charging profile.